I am running KMail 4.10.5, imported certificate with root CA Verizon and added the root CA to the ~/.gnupg/trustlist.txt file.

As seen on S-Mime config screen it is not possible to pick up my certificate for S-MIME signing, however for encryption it al works well. Even when there is nothing in the search fields the list remains empty though many certificates are imported already.

UPDATE: It turns out that the problem was gpg-agent not starting up automatically with KDE session, found it KDE bug reporter.

Edit file /etc/kde/startup/agent-startup.sh to start gpg-agent as daemon with KDE session. Just uncomment the lines in the file.